1. Field of the Invention
The present invention relates to light emitting/lighting devices and systems, and in particular, it relates to a control method and apparatus for the light source in projection systems.
2. Description of the Related Art
A typical projection system includes a light source system which includes several primary color light sources, e.g. including but not limited to red, green, and blue light sources generating red (R), green (G) and blue (B) light respectively. The projection system further includes a light valve for modulating the red, green and blue light to form a color image, and a projection lens for projecting the modulated red, green and blue light onto a screen to form the image. The red, green and blue light can be combined and delivered to the light valve in a manner of but not limited to being directed by optical fibers to a color-combining prism. The light valve may be a micro-electro-mechanical (MEMS) system such as a spatial light modulator (SLM) based on the Texas Instrument (TI)'s Digital Light Processing (DLP) technology. The image signal processor (ISP) of the projection system controls the light source system by a driving signal and provides the image data for the SLM, for example, a digital micromirror device (DMD). For example, when red light source is on to generate a red light, the ISP provides the red image data for the SLM. Similarly, when the green/blue light source is on to generate green/blue light, the ISP provides the corresponding green/blue image data for the SLM. If all the red, green and blue light sources are on, grey image data will be provided for the SLM. The light source system may also provide a feedback signal for the ISP, which forms a feedback loop together with the driving signal to keep the chromaticity and luminance of the light sources stable. The modulated light final is projected by a projection lens onto a screen to form the image.
For its relatively low cost, an Ultra High Performance (UHP) lamp is generally used in current projection systems as a light source to generate white light. The white light passes through a color-adjusting apparatus, for example, a segmented color filter which can respectively transmit red, green and blue primary color light necessary for projection. This solution has a number of disadvantages: UHP lamps need a high startup voltage which is usually several thousand volts; UHP lamps have a long switching time between on and off states; the output colored light has a small color gamut; and UHP lamps have a short lifetime of about 2,000 hours.
With its technology development, light emitting diodes (LEDs) are used more and more in light source systems without the above disadvantages. For example, different colored LEDs are used to generate different primary color light beams such as red, green, blue and yellow light, which have the advantages of a large color gamut and long lifetime (about 20,000 hours).
To reduce the cost and increase the luminance of output light from the light source, current LED light source systems usually have single color LEDs and a color-adjusting apparatus with wavelength conversion materials to generate different colored light. Referring to the color wheel as an example of the color-adjusting apparatus, the color wheel is usually composed of several segments containing different phosphors. When the wheel rotates, the LED light illuminate the wheel's different segments at different time, thus emitting different single color light sequentially corresponding to different phosphors of the segments. For example, blue LEDs can be used to generate necessary red, green and blue light in three colors' projection display systems.
To further increase the luminance of the output light, UV LEDs can be used together with blue LEDs to excite the phosphors in a LED based light source. A schematic structure of such a light source is shown in FIG. 1. In this structure, the first light emitting source 2 is blue LEDs, and the second light emitting source 3 is UV LEDs. Light coupling/conducting apparatus 6 collects the light from the first and second light emitting source 2 and 3, and directs the collected light to a rotary color wheel 1 with segments. As shown in FIG. 3, the segmented wheel may be divided into four segments (for example, a, b, c and d segments) which contain red (R), green (G), blue (B) phosphors respectively. Two segments of them both contain the same blue phosphor. Therefore, the wheel has four segments with three different wavelength conversion characteristics. The light coupling/conducting apparatus 6 may be a multi-optical fiber that includes several optical fibers. As shown in FIG. 1, one end of the multi-optical fiber 6 is two optical fibers 62 coupling and collecting the light from the first light emitting source 2 and second light emitting source 3, and the other end is one optical fiber 63 combined by the two optical fibers directing the collected light to the color wheel 1.
A more compact structure of the current light source systems with lower cost is shown in FIG. 2. A dichroic filter 5 that can transmit blue light and reflect UV light is used in place of the light coupling/conducting apparatus 6 of FIG. 1, to combine the blue excitation light 41 and UV excitation light 42 into one light path directed to the color wheel 1. In order to fully use combined excitation light, a lens 7 is usually inserted in the light path to focus the light onto the segmented color wheel 1.
There is a disadvantage of the current technology described above. For example, when the first light emitting source 2 is blue LEDs, the blue segments of the wheel in FIG. 3 contain no phosphor and can transmit blue light, directing the light from the blue LEDs for output light of the light source, therefore, the UV light from the second light emitting source 3 is not used here. Because UV light can cause harm to human eyes, a filter preventing the unused UV light from passing through are necessary in the light path, for example, after the color wheel. It is obvious that such systems waste some UV light and have additional cost.